The present invention relates to a method and apparatus for removing acid gases including hydrogen sulphide and carbon dioxide from a gas stream. In particular, it relates to the treatment of gas streams emanating from produced or stored unrefined crude petroleum, or from a produced natural gas stream or from other sources.
Crude oil is a mixture of many hydrocarbon liquids and gases entrained or dissolved in the liquids. Such gases include lower alkanes (methane, C1; ethane, C2; propane, C3; butane, C4) as well as carbon dioxide and hydrogen sulphide. The liquids are typically mid-range hydrocarbons, (C5 to C7) and higher-level hydrocarbons (C8 and above). Mid-range hydrocarbons (pentane, C5, hexane, C6 and heptane, C7) are normally in a liquid state at room temperature but have significant vapor pressures at ambient temperatures and therefore contribute to the vapor phase in the mixture of gases above the liquid crude. Other gases entrained in oil include hydrogen sulfide. Landfill gas contains mainly CO2 and methane. Emissions from coal-fired and most oil refining operations contain mainly CO2 and SO2.
Some of the gases in crude petroleum, especially hydrogen sulphide, are extremely toxic and must be separated from the crude oil before shipment to a refinery. Typically, hydrogen sulphide in crude oil storage tanks is disposed by partial incineration in a flare stack, in the field. The flaring process burns the hydrocarbons, hydrogen sulphide, hydrogen, and any other flammable vapors in the produced gas stream. Unfortunately most flaring processes involve low temperature, low pressure and incomplete combustion of hydrocarbon gases in particular, as evidenced by the orange color seen in flare stack flames, which is typical of incandescent carbon. Since the gas stream entering the flares typically contains a flame retardant in the form of carbon dioxide, oxygen from the air is less able to react with the hydrocarbons from the gas stream. This results in only partial combustion of some of the flared gases producing carbon monoxide, carbon plus water and sulfur dioxide from hydrogen sulphide combustion. Complete combustion or incineration of acidic hydrocarbons produces sulfur dioxide, water and carbon dioxide. Considering how noxious the by-products produced by the flaring process are, flaring and/or incineration are only marginal improvements over simply letting the gas stream into the atmosphere. Both carbon dioxide and sulfur dioxide, each for their own reasons, are serious industrial pollutants today worldwide.
Produced natural gas at the wellhead often contains hydrogen sulphide. Small amounts of hydrogen sulphide are routinely removed from produced gas before distribution to customers by passing it through heated amine scrubbers at a significant capital and operating cost. Large amounts of hydrogen sulphide and other undesirable gases in produced gas are much more expensive to remove. Often gas wells with high H2S and/or CO2 to hydrocarbon gas ratios are left dormant because of the high cost of H2S and/or CO2 removal.
The use of ammonia to remove hydrogen sulphide is well known, however, it has not been considered an efficient process because the high disassociation pressure of the resulting ammonium sulphide required large amounts of ammonia and water to maintain the reaction equilibrium in favour of ammonium sulphide.
Alternatively, as disclosed in U.S. Pat. No. 2,866,679 issued to Lamont, Dec. 30, 1958, a solution to this problem involves maintaining the ammonia-hydrogen sulphide reaction at very high pressure, in the order of 3500 psia. Working at such high pressures has its disadvantages as it requires pressure-rated vessels and piping and introduces additional cost, complexity and risk to the operation.
It is also known to remove hydrogen sulphide from gases by contacting them with iron oxide in the form of pellets, powder or iron oxide sponge. Iron sulphide is formed which is then reacted with oxygen to regenerate the iron oxide and form elemental sulfur. This process does not affect or remove other acid gases such as carbon dioxide. Also, the removal of spent iron oxide sponge impregnated with sulfur is a difficult and noxious task.
Therefore, there is a need in the art for a method and apparatus for efficiently removing acid gases including H2S and CO2, which does not involve incineration and is cost-effective. Simultaneous removal of H2S and CO2 is particularly desirable due to CO2 being a serious pollutant, and a flame retardant in any combustion process. It would be advantageous if the apparatus were compact and portable allowing installation and practice of the method on a relatively small scale. Such a method and apparatus may permit the reclamation of low volume gas wells previously deemed uneconomic.